Coupled plasmonic systems are of great interest and have many applications such as information processing and sensing. By choosing proper geometric configurations of coupled plasmonic systems, one can obtain various optical properties. However, some interesting and important effects could not be described by earlier methods. We develop an improved method for coupled plasmonic nanoparticle systems that maps geometric configurations to optical properties more accurately. With the improved method, we realize a low-loss cavity of metallic nanoparticles through a proper geometric configuration, and we find a limit to the loss in the metallic nano-cavity. We also use this method to realize an exceptional point and exceptional nexus in a hybrid plasmonic system. Finally, we predict asymmetric coupling, which leads to chirality and directional energy transfer.
In experiments, we demonstrated that luminescence quantum yield of single gold nanorods illuminated by continuous wave laser at wavelength of 532 nm depends on the excitation polarization, while that excited by 633 nm laser does not. The electrons in sp-band dominates the luminescence process when the 633 nm laser is applied, resulting in a constant quantum yield under different excitation polarizations. When the 532 nm laser is applied, both the electrons in d-band (interband transition) and sp-band (surface plasmon) involve in the luminescence process. The variation of quantum yield by the 532 nm laser is resulted from different efficiency of d-band interband transition and sp-band plasmon conversion into luminescence. Furthermore, we found that plasmon modes coupling effect can modulate strongly the plasmon emission efficiency by comparing the luminescence of two sets of the nanorods with different size. And smaller size GNRs often results in higher quantum yield of interband transition. These findings make a step to understand the luminescence process of plasmonic nanostructures and point out a rule to control it through plasmon mode coupling effect.
Detecting a single-molecule fluorescent spectrum with a low quantum yield needs to enhance the signal greatly overcome the noise disturbance. Based on plasmon-enhanced fluorescence method with single gold nanorods antenna, we measured the emission spectra of a single methylene blue or crystal violet molecules with an effective integration time of ~2.14 milliseconds at room temperature. These transient spectra have revealed the single-molecule spectral shape and intensity fluctuations. We found that the transient spectral shape is dominated by the molecule, while the plasmon resonance determines the averaged spectral shape. These fluctuations can be understood as transitions between metastable minima in the molecular potential-energy surface, as supported by complementary ab initio simulations.
In the surface-enhanced fluorescence (SEF) process, it is well known that the plasmonic nanostructure can enhance the light emission of fluorescent emitters. With the help of atomic force microscopy, a hybrid system consisting of a fluorescent nanodiamond and a gold nanoparticle was assembled step-by-step for in situ optical measurements. We demonstrate that fluorescent emitters can also enhance the light emission from gold nanoparticles which is judged through the intrinsic anti-Stokes emission owing to the nanostructures. The light emission intensity, spectral shape, and lifetime of the hybrid system were dependent on the coupling configuration. The interaction between gold nanoparticles and fluorescent emitter was modelled based on the concept of a quantised optical cavity by considering the nanodiamond and the nanoparticle as a two-level energy system and a nanoresonator, respectively. The theoretical calculations reveal that the dielectric antenna effect can enhance the local field felt by the nanoparticle, which contributes more to the light emission enhancement of the nanoparticles rather than the plasmonic coupling effect. The findings reveal that the SEF is a mutually enhancing process. This suggests the hybrid system should be considered as an entity to analyse and optimise surface-enhanced spectroscopy.
Light emission from single gold nanorods excited by continuous wave lasers was investigated by using ultra-narrow-band notch filters to obtain their complete spectral shape. The spectral profile of Stokes emission can be fitted by a Lorentzian line shape and that is dominated by localized surface plasmon resonance. Moreover, a clear anti-Stokes emission band can be always observed under different excitation wavelengths. The spectral shape of anti-Stokes emission can be fitted well with a Fermi-Dirac like line shape. Electron Fermi-Dirac distribution should influence the spectral shape of anti-Stokes emission for both interband and intraband transitions. It was also found that the intensity of anti-Stokes emission increases more rapidly in comparison with that of Stokes emission as illumination power increases on resonant excitation. This phenomenon can be understood as the temperature dependent of the electron Fermi-Dirac distribution due to photothermal effect.
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